Typically, a secondary battery in which the amount of energy (electrical power) that is charged and discharged is great is used while maintaining safety by means of, for example, a protective circuit. In particular, a lithium-ion secondary battery has higher energy density than other secondary batteries of the same battery capacity and must therefore be used together with a protective circuit.
The main functions of a protective circuit are over-charge protection and over-discharge protection. Over-charging and over-discharging are normally detected by monitoring the voltage across the terminals (positive terminal and negative terminal) of the secondary battery. As a result, the voltage across the terminals of a secondary battery must be measured accurately to prevent over-charging and over-discharging.
In addition, management of the amount of electrical power accumulated in the secondary battery (hereinbelow referred to as residual battery power) is important when using a secondary battery. The inter-terminal voltage of a secondary battery can be used in the management of residual battery power, and based on the inter-terminal voltage that is measured in the protective circuit, a processor such as a CPU (Central Processing Unit) can estimate the residual battery power from the relation between the inter-terminal voltage and the residual battery power that has been found in advance.
A configuration that employs an operational amplifier is generally known as a circuit for measuring the inter-terminal voltage of a secondary battery of this type (hereinbelow referred to as a voltage measuring circuit). An example of such a circuit is shown in FIG. 1.
The voltage measuring circuit of the background art shown in FIG. 1 is of a configuration in which the inter-terminal voltage of a secondary battery is voltage-divided using a plurality of fixed resistors (resistors in which resistance is fixed) and the voltage-divided voltage is then supplied to a processor by way of an impedance conversion circuit made up by operational amplifier 3. The processor calculates the inter-terminal voltage of the secondary battery based on the output voltage of operational amplifier 3 and the voltage-division ratio realized by the plurality of fixed resistors.
In addition, a configuration, such as described in Patent Document 1, is also known as a circuit that uses an operational amplifier to measure the inter-terminal voltage of a secondary battery. In Patent Document 1, a configuration is described in which the terminal voltages of the positive electrode and negative electrode of a secondary battery are each detected by a respective differential amplifier made up by an operational amplifier and the inter-terminal voltage of the secondary battery is supplied from the differential amplifiers to an A/D (Analog to Digital) converter.
In recent years, secondary batteries are also being used as the power supply of electric automobiles or hybrid automobiles, and further, attendant to the problem of global warming, the use of secondary batteries is being investigated for the storage of electrical power that is generated by a renewable power supply such as solar batteries that are being introduced for the realization of a low-carbon society. In other words, the uses of secondary batteries have expanded with the increase in their capacity, and secondary batteries are coming into use as, for example, the drive apparatus such as a motor that is mounted in an electric automobile or hybrid automobile or the power supply of a stationary apparatus such as an inverter that converts power for enabling supply of electrical power that has been generated by a renewable power supply to an electrical power system.
A drive apparatus or a stationary apparatus becomes the source that generates noise that affects control over a significant amount of electrical power. As a result, the measurement accuracy of the inter-terminal voltage of the secondary battery falls due to this noise. There is the additional concern that the noise generated by the drive apparatus or stationary apparatus may cause malfunctioning of the processor such as a CPU that is incorporated in the protective circuit.
In the voltage measuring circuit of the background art that is shown in FIG. 1, the negative potential of a secondary battery that supplies electrical power to a drive apparatus or stationary apparatus (not shown) that is the source that generates noise shares the ground potential with the processor, whereby the concern arises that the accurate measurement of the inter-terminal voltage of the secondary battery is prevented by the superposition of noise on voltage that has been voltage-divided through the use of a plurality of resistors.
On the other hand, in the voltage measuring circuit described in Patent Document 1, a differential amplifier is used to detect each of the terminal voltages of the positive electrode and negative electrode of the secondary battery, and the common-mode noise generated at the positive electrode and negative electrode of the secondary battery can therefore be prevented. As a result, the effect resulting from noise that is generated in a drive apparatus or stationary apparatus is reduced to a greater degree than the voltage measuring circuit shown in FIG. 1. Nevertheless, noise that is generated as a result of operation of a drive apparatus or stationary apparatus is not limited to only common-mode noise, and the voltage measuring circuit therefore preferably has a configuration which has high resistance to noise.